My driver has been using a little more fuel than usual, so I looked into it. Tire PSI and air filter both checked out. I've been having the Check Engine light come on and off lately, and in the past few days it just stayed on. So I hooked up the simple Actron OBDII scanner and got some codes back.

P0128 - Coolant Temp Below T-stat Regulating Temp

I think the likely culprits are the Temp Sensor and the T-stat itself. The car never overheats, so I'm guessing the T-stat is working but I've heard they "fail open" on these cars. I have a feeling the Temp Sensor might not be working, the needle doesn't seem to move up from zero lately. Any way to know for sure without removing the T-stat?

P0141 - 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

This one is pretty straight forward, one of the downstream 02's is malfunctioning. The service interval is 100k and the car has 111k so it's pretty reasonable to assume the sensor is just toast. But before I spend the $70 for a new one, is there anything simple that might be making an otherwise good sensor malfunction?

Also, does anyone know off hand which side of these engines is considered Bank 1?

And would either of these problems cause a noticeable change in fuel mileage? I'm only getting 21mpg mixed city/hwy, mostly hwy. This thing used to get 25-27, sometimes 30mpg on long highway trips (Vegas runs :D ) Maybe it's just showing its age though. I just looked online and some sources rate it at 22 city, 30 hwy, but recent reviews show 20/25 and 22 mixed.

Cold water temps will tell the computer it is cold outside and use more fuel. Lots of the late model performance guys use coldet t-stats. Depending upon which way the O2 sensor is sensing it may state a lean condition even though there is not and tell the computer to use more fuel.

Bank 1 is the side with the #1 cylinder, you need a tstat, it's not letting the engine come up to temp, your temp sender is most likely fine.

P0128 - Coolant Temp Below T-stat Regulating Temp

I think the likely culprits are the Temp Sensor and the T-stat itself. The car never overheats, so I'm guessing the T-stat is working but I've heard they "fail open" on these cars. I have a feeling the Temp Sensor might not be working, the needle doesn't seem to move up from zero lately. Any way to know for sure without removing the T-stat?

I agree on the thermostat not working. Your car has two temp sensors. One for the engine computor and one for the dash gauge. If you have a code for cold operating temperature and the dash gauge also reads low the engine is definately not running warm enough. It would be very unusual if BOTH temp sensors failed. The engine running cold will use more fuel.

Paul

I agree on the thermostat not working. Your car has two temp sensors. One for the engine computor and one for the dash gauge. If you have a code for cold operating temperature and the dash gauge also reads low the engine is definately not running warm enough. It would be very unusual if BOTH temp sensors failed. The engine running cold will use more fuel.

Paul

Thanks guys. New T-stat and new 02, there goes a nice chunk of change. Hopefully the fuel milage will pick up some.

So I the #1 cylinder on these engines in the same place as the older GM V8/V6 engines? Meaning, if it was facing the same way as an SBC, would 1 be in the same corner?

I have sent you an email about your trouble codes. For the O2 snsor code it gives a list of codes that may also set code P0141. The code P0128 is one of those codes. To me that means that the O2 sensor is probably OK and the P0141 code was set also due to the engine running too cool.
Hope this helps.

Paul

I have sent you an email about your trouble codes. For the O2 snsor code it gives a list of codes that may also set code P0141. The code P0128 is one of those codes. To me that means that the O2 sensor is probably OK and the P0141 code was set also due to the engine running too cool.
Hope this helps.

Paul

I appreciate the help Paul! I'll work on the cooling system this weekend and see what happens.

Thanks!